The present invention relates to an image transfer type thermosensitive recording medium comprising a substrate and a double-layered thermal transfer ink layer, which is capable of recording images clearly on a recording sheet even though it has a rough surface, and has excellent preservability.
An image transfer type thermosensitive recording method is widely used as a method of recording images on plain paper by using a simple apparatus. However, the image quality obtained by this method largely depends upon the surface quality of an image receiving sheet for recording images thereon. As a matter of course, when the receiving sheet has a rough surface, it is difficult to print clear images thereon.
In order to improve the conventional image transfer type thermosensitive recording method in the above respect, for instance, the following methods have been proposed: subjecting printed images to thermal treatment as proposed in Japanese Laid-Open Patent Application No. 58-76276; using magnetic force as auxiliary means for image transfer at the time of image transfer as proposed in Japanese Laid-Open Patent Application No. 52-96549; using electrostatic force as auxiliary means for image transfer at the time of image transfer as proposed in Japanese Laid-Open Patent Application No. 55-65590; adding a large amount of an oily material to an image transfer layer, thereby decreasing the melting viscosity of the image transfer layer at the time of image transfer as disclosed in Japanese Laid-Open Patent Application No. 60-25762; increasing the thermal sensitivity of an image transfer layer by adding thereto a heat decomposable material as proposed in Japanese Laid-Open Patent Application No. 60-82389; and increasing the thermal sensitivity of an image transfer layer by adding thereto a thermally expansive material as proposed in Japanese Laid-Open Patent Application No. 60-25762.
In addition, a multi-layered thermofusible ink layer has been proposed for improvement of printed image quality, with the melting point of each layer gradually changed, with addition of a pigment to any of the layers as in Japanese Laid-Open Patent Application No. 59-224392. Furthermore, it has been proposed to form a thermofusible layer without containing therein a colorant on a thermofusible ink layer as in Japanese Laid-Open Patent Application No. 60-97888.
However, in the above-mentioned recording methods, a melted ink is transferred to an image receiving sheet when images are recorded thereon. Therefore, when the surface of the image receiving sheet is not smooth enough, the obtained printed image becomes poor in image quality. In other words, any of the above-mentioned methods still has the shortcoming that printed image quality depends upon the smoothness of the receiving sheet.
If such an ink is employed that comprises as the main component a resin which can be fused to exhibit adhesiveness while maintaining a mechanical strength to some extent, without becoming a liquid having a low viscosity, when thermal energy is applied, the ink may cover any unevenness of the surface of a receiving sheet and make it smooth when applied to the receiving sheet, even if it has a rough surface. The result will be that high printing quality may be obtained though the receiving sheet has a rough surface.
However when an ink containing such a resin is used, a larger quantity of thermal energy will be required for printing as compared with an ink containing a conventional wax, so that a support film having particularly high heat resistance may be required. In addition, the life of a thermal head for use with such ink may be short because of the use of such large quantity of thermal energy, and the accumulation of heat in the thermal head will also become a problem when used in practice.
Further thermosensitive image transfer media have been proposed in Japanese Laid-Open Patent Applications Nos. 60-239284 and 60-239285, which are capable of yielding images with clear background without reduction of image resolution. These thermosensitive image transfer media comprise a heat resistant support and an ink layer formed directly thereon, which ink layer contains a solvent having high boiling point, such as phosphate ester, phthalate ester, animal oils, vegetable oils, mineral oils, higher fatty acids, and higher alcohols. These thermosensitive image transfer media, however, have the shortcoming that the thermal sensitivity is so low that it is difficult to carry out high speed printing with application of low thermal energy. Further, in order to prevent the reduction of image resolution, it is necessary to add a large quantity of any of the above-mentioned solvents having high boiling points to the ink layer. The addition of such solvents degrades the preservability of the thermosensitive image transfer media and makes it difficult to perform smooth coating of the ink layer on the support.
Further, there is proposed in Japanese Laid-Open Patent Application No. 60-187593 a thermosensitive image transfer material comprising a support, and a first ink layer and a second ink layer which are successively formed on the support, with addition to the first ink layer of a mineral oil such as machine oil, or a vegetable oil such as castor oil, olive oil and rape oil. This thermosensitive image transfer material, however, has the shortcomings that images with clear background cannot be obtained, and the preservability of the material is poor, although excellent line images and solid images can be obtained by the first ink layer.